The goal of telecommunication industry is in general the highest possible amount of data transfer in shortest possible time. With the ever increasing number of users and complexity of services there exists a daily increase in demand for the amount of data that can be transferred.
In the art there are variety of solutions that can be applied to achieve high data rate transfer over longer distances. Optical communications, that were topic of intense research and development in past 20 years, make it possible to achieve a bandwidth not comparable to other technologies. The solutions that are used today in communications over shorter distances are mainly based on multimode fibers. The advantage of multimode fiber lies mainly in the ability to couple this fiber with simple and more cost effective sources. In the past these sources were mainly LEDs with the wavelength around 850 nm. Lately, low cost laser diodes with a vertical resonator (VCSEL) have appeared in the market that enable effective coupling between optical fibers and they also achieve high modulation rates.
Conventional multimode fiber has been known for over 25 years and is standardized by international standards such as ITU-T G.651. According to the most recent recommendations such as OM3, multimode fibers can achieve bandwidths of up to 1.5 GHz.km when all modes of the fiber are excited. But even when the bandwidth of 1.5 GHz.km is achieved, differential mode delay proved to be still over 1 ns/km, which limits further increase of bandwidth. The development of low cost vertical cavity lasers (VCSEL) has lately resulted in a new concept where laser selectively excites only the lower order modes of optical fiber. This results in lower delay between fastest and the slowest excited modes, that allows for achieving the bandwidths in the excess of 1.5 Gbit.km. This approach has been the topic of intense research that lately resulted in the adoption of IEEE 802.3z and IEEE 802.3ae standards. However, the selective excitation of modes in graded index fiber also brings a range of drawbacks. Optical connectors of insufficient quality, bad splices, certain optical components cabling and process of applying the cables can induce mode coupling between the modes that can result in sudden and unexpected degradation of fiber bandwidth. The systems that base on selective excitation also require more complex field test equipment. In addition the selective excitation of modes requires better and narrower range of tolerances for the components that reduces the cost efficiency of the transmission systems. The potential main drawback of selective excitation of the modes in the future can be the lack of compatibility of such concept with the emerging technology of VCSEL arrays. The VSCEL array is composed of larger number of laser diodes with different wavelengths that are integrated on the same chip. Such configuration can allow for efficient wavelength division multiplexing. VCSEL array can be efficiently coupled to an optical fiber under the condition that the fiber core area in which the light needs to be coupled is sufficiently large. Larger VCSEL array therefore cannot be coupled with the multimode fiber while exciting only a limited number of modes.
There is an oblivious need to have a multimode fiber with large bandwidth even when an arbitrary or entire set of modes is excited.
There exists a variety of systems that enable the increase of bandwidth of multimode fibers. As already described, most of those systems are based on selective excitation of modes in multimode fibers. There are also individual non-standard solutions that are based on selective excitation of the modes as described for example in U.S. Pat. No. 6,580,543 and U.S. Pat. No. 6,330,382. There are solutions that enable archiving the bandwidths over 1.5 GHz.km in an restricted launch but at the same time enable bandwidths over 500 MHz.km while exciting the entire set of modes. Such solutions are presented in U.S. Pat. No. 6,434,309, U.S. Pat. No. 6,438,303, U.S. Pat. No. 6,618,543 and U.S. Pat. No. 6,724,965. Dispersion compensation according to U.S. Pat. No. 6,363,195 has also been proposed but its efficiency is limited.
Significant efforts have been also made to develop fiber index profiles that yield high bandwidth to equalize the delay times of high order modes in a multi-mode fiber and to compensate for the center dip. Those efforts were mainly done in the early era of optical fibers. For example in Geshiro et al., “Truncated Parabolic-Index Fiber with Minimum Mode Dispersion,” IEEE TRANS. MICROWAVE THEORY AND TECHNIQUES, Vol. MTT-26, No. 2 (February 1978), at p. 115, a parabolic index profile is combined with a cladding jump, which leads to higher bandwidths than with a parabolic profile with no cladding jump. U.S. Pat. No. 6,292,612, discloses MM fiber having a refractive index profile that differs from a conventional α-type profile by at least one of the following: i) a step formed in the index profile at the core/cladding boundary, in conjunction with a linear correction; (ii) a ripple near the core/cladding boundary, in combination with a linear correction, with or without an index step; and iii) an annular ridge formed in the index profile with a center dip. In Stolz and Yevick “Correcting Multimode Fiber Profiles with Differential Mode Delay”, J. Optical Communications, vol 4 (1983), no 4 pp. 139-147 trimming of the MMF core edge to reduce the highest order modes differential delay has been proposed.
The idea of reducing the higher order mode delay by extension of graded index core beneath the cladding level was first proposed in K. Okamoto and T. Okoshi, “Analysis of Wave Propagation in Optical Fibers Having Core with a α-Power Refractive-Index Distribution and Uniform Cladding” IEEE Transactions on Microwave Theory and Techniques, 24(7), pp. 416-421, March 1976. In this work the α-profile multimode fiber profile was extended below the cladding level (e.g., outside the core/cladding boundary region), with a negative cladding jump. The supporting analysis demonstrated that the absolute value of negative extension of the α-core beneath the cladding level needs to be approximately of the same depth as the absolute difference between the maximum refractive index of the core at its center and the cladding level. In case of common silica telecommunication multimode fiber with index Δ=1%, such negative refractive index core extension beneath the cladding would need to be approximately −1%. This is a condition that is not technically and economically compatible with current manufacturing techniques for silica-based fibers. However, in plastic optical fiber where negative index differences of the profile with the respect to the cladding are easy to achieve, this approach has been already successfully applied, see T. Ishigure, H., K. Ohdoko, and Y. Koike, “High-Bandwidth Plastic Optical Fiber With W-Refractive Index Profile”, IEEE Photonics Technology Letters, 16 (9), pp. 2081-2083, September 2004. Further efforts were conducted to develop this concept in Katsunari Okamoto and Takanori Okoshi in “Computer-Aided Synthesis of the Optimum Refractive-Index Profile for a Multimode Fiber” IEEE Transactions on Microwave Theory and Techniques, 25(3), pp. 213-221, March 1977. A trial and error algorithm was used to automatically search for an optimum fiber profile shape that includes the extension of the core beneath the cladding level. The result of numerical optimisation was an optimal profile, which is reported to be a smoothed W-shaped profile (e.g., FIG. 1 thereof. This synthesized profile was similar to α-profile fiber in the region with positive relative refractive index difference, however in the region near the edges of the core where the index difference was below the cladding level, the smoothed shape deviating from α-alpha profile was characteristic. The maximum absolute value of the relative index difference of the core extension and the cladding level was still in the best case one half of the absolute difference between the maximum refractive index of the core at its center and the cladding level. In case of standard 50 μm fiber the index maximum relative negative index difference of the profile would need to be below −0.5% while the region would require precise shaping in accordance with the results of numerical optimization process. While such profiles may be advantageous in leading to high bandwidths, they are barely incompatible with the manufacturing process for silica-based fiber. It is know from the literature that achieving negative index difference in silica fiber manufacturing process close or below −0.5% is very difficult and uneconomical, especially if precise control of the profile is required. In addition, it is believed that such profiles may lead to leaky modes as the regions with the negative index difference are deep and wide and provide good isolation of leaky modes from the cladding.